The present invention relates to a super precise position detector, and more particularly, to a position detector for detecting a relative position between a mask and a wafer in a stepper X ray exposure apparatus in the order of 0.01 .mu.m by employing sector Fresnel zone plates.
In general, in a stepper X ray exposure apparatus, X ray microanalyzer and the like, it is impossible to set up an optical microscope right over an object to be analyzed, processed or worked in an ordinary manner so as to observe a desired point on the object, since the area right over the object is in the passage of X rays, electroncurrent and the like. For this reason, an area to be observed is observed obliquely with an ordinary optical microscope. At this time, due to the depth of a focus, the area coming into the field of vision is extremely limited. In addition, when a lens group is disposed in parallel with the surface of an area to be observed to obliquely observe an object to be observed, an image is formed by an oblique ray, resulting in that aberration increases and an image to be observed becomes indistinct.
With regard to a stepper X ray exposure apparatus of the above-mentioned arrangement, I have proposed a novel position detector in Japanese Parent Application No. Sho 63-162915 bearing the title "a position detector employing double linear Fresnel zone plates under illumination of multiple wavelength light". This position detector is to detect a relative position between a mask and a wafer which are spaced at a minute distance from each other in the direction of the optical axis of exposure X ray, in the direction perpendicular to the optical axis.
In this position detector, a mask and a wafer are respectively provided with a linear Fresnel zone plate (hereinafter referred to as LFZP) on which alignment marks are formed. The LFZPs are simultaneously illuminated by light rays of a plurality of wavelengths in the same direction from the upper in slant. An objective lens is disposed at an angle with the surface of the LFZPs on the opposite side with respect to the normal of LFZP. The lens has such an axial chromatic aberration that focal planes of the lens to light rays of a plurality of wavelengths agree with principal focal planes of the LFZPs by Fresnel diffraction to the plural wavelength rays which planes are different in position with each of the light rays, respectively. The Fresnel diffraction images on the principal focal planes of the LFZPs which planes are different in position to each of the plural wavelength rays incident upon the LFZP, are respectively focused through the objective lens on the same image forming planes of the objective lens in superposing relationship into a straight line. The Fresnel diffraction image in a straight line is converted into an electric signal by a linear sensor disposed on the image forming plane by a linear scanning operation in the direction perpendicular to the longitudinal direction of the image. A cylindrical lens is disposed between the objective lens and the linear sensor such that the Fresnel diffraction image in a straight line is compressed in the longitudinal direction thereof and is formed on the linear sensor. Then, a signal thus obtained by the linear sensor is handled to detect a relative position between the alignment marks on the LFZPs.
The principal focal plane (62) of the LFZP, however, as shown in FIG. 1, is parallel to the surface of LFZP, so that the principal focal planes of the alignment marks are parallel to the surface of the mask and wafer.
With such arrangement, to make a positional alignment servo operation possible during X ray exposure, it is necessary to enlarge a detection angle of a detection optical system disposed downstream of the objective lens with respect to the normal of the wafer (mask) plane. A large detection angle, however, deviates largely from an angle of 90.degree. between the optical axis of the detection optical system and the focal planes of the alignment marks to reduce the image forming area of the detection optical system with the result increasing distortion of an image due to an oblique image formation. As a result, it is very difficult to satisfactorily detect a light intensity of Fresnel diffraction image by the linear sensor with a good linearity and accuracy. As a solution to this disadvantage, it is conceivable to apply the improvement of a detection optical system proposed in Japanese Patent Publication No. Sho 44-23794, for example. The proposal has the following structure. In a detection optical system which observes an object surface from the upper right thereof, in order to form a real image perpendicular to the optical axis of an eyepiece on the optical axis, an objective lens is inclined with respect to the observing surface of the object and the eyepiece. Alternatively, a detection optical system is constructed so as to interpose a parallel ray portion within part of the optical path of the detection optical system. However, this system has the following disadvantages.
1 Design and manufacture of the optical system are cumbersome. PA0 2 Optical performance deteriorates. PA0 3 The resolving power falls due to decrease in optical performance. PA0 4 The detection angle of the optical system with respect to the surface of an object is limited. PA0 1 Pattern measuring method (contrast method, edge detection method), PA0 2 Method utilizing a diffracted light ray. These methods each have their advantages and disadvantages and there is little reason to choose between them. The present invention, however, provides a position detector employing a hybrid alignment system combining the advantages of the above 1 and 2. PA0 (1) Is is possible to make an oblique detection with an extremely simple optical system without interfering with an exposure X ray. PA0 (2) Since an oblique detection angle is determined by the design of an SFZP, a resolving power of the objective lens can be increased independently of the detection angle, so that a highly accurate detection is possible. PA0 (3) A servo control is enabled during an X ray exposure operation to detect alignment marks provided within the X ray exposure area. PA0 (4) Dependence upon process is extremely reduced. PA0 (3) There is no effect of variation in a gap between an X ray mask and a wafer and dependence upon the gap is extremely reduced. PA0 (6) High speed detection is possible. PA0 (7) The size of marks is reduced. PA0 (1) It is possible to perform a detection by largely inclining an objective lens of the detection optical system. PA0 (2) It is possible to incline the detection optical system so as to be outside the X ray exposure zone. PA0 (3) It is possible to detect the alignment marks during an X ray exposure operation. PA0 (4) It is possible to use the alignment marks within the X ray exposure zone. PA0 (5) In order to raise an optical resolving power of an objective lens of the detection optical system, the numerical aperture of the objective lens should be increased. On the other hand, the objective lens can be largely and simply inclined by employing SFZPs without interfering in the X ray exposure zone. Consquently, it is possible to set a limit of the numerical aperture which can be set with an oblique angle, to an very high value. PA0 (6) Since it ts possible to detect the alignment mark, which is provided within the X ray exposure zone, even during the exposure operation without interfering with the exposure X ray, a position of a mask/a wafer can be detected and corrected without evacuating the detection optical system during the X ray exposure operation. Since it is possible to effect a step repeating exposure on the whole surface of a wafer, an accuracy of alignment and a throughput operation can be remarkably improved. PA0 (7) Although the shape of alignment marks becomes somewhat complicated as compared with a linear Fresnel zone plate because a factor of angle is added to the alignment marks, the shape is much simpler than a circular Fresnel zone, so that a pattern change due to a process change seldom occurs. PA0 (8) Since the oblique incidence angle is determined by a SFZP and the detection optical system does not involve a special technique in design regarding the oblique incidence and difficulties in manufacturing, the design and manufacturing are simplified and hence a cost is reduced. PA0 (9) Both the illumination and detection systems can be constructed with a simple optical system. PA0 (10) The scope of detection is much wider than that of a diffraction light system. PA0 (11) In the detection optical system, PA0 (12) A detection accuracy is influenced by the nature of an image spot with a Fresnel zone plate as follows. PA0 In contrast, an oblique incidence is utilized in a position detector employing SFZPs, so that it is possible to completely shut out a spot produced by a ray passing through a mask by providing a window zone and a shielding zone, for example. There is no effect of variation in a gap between a mask and a wafer, resulting in a stable and accurate detection.
Consequently, it is not preferred even if the above proposal is applied to the foregoing position detector employing double linear Fresnel zone plates.